The proposed investigation will concern the initial events involved in the activation of resting B lymphocytes to a state of antibody synthesis. The function of two classes of surface molecules in B cell triggering will be considered; these molecules are: 1. surface immunoglobulin (sIg), and 2. alloantigens selectivity expressed on some or all B lymphocyts (Lyb antigens). Previous work on the relative importance of sIgM and sIgD for the triggering of developing B lymphocyte subpopulations by thymus-independent (T-I) antigens will be extended to include studies on the quality of signals, (proliferative, differentiative, or both) delivered by T-I antigens binding to either sIgM or sIgD alone. In addition, the role of Lyb antigens in the activation of B cells will be sought, with particular emphasis on Lyb 7, a molecule expressed on a subpopulation of B lymphocytes which respond to a subclass of T-I antigens. The interaction of sIg and Lyb antigens will be investigated to determine their association in the cell membrane and their behavior upon antigen binding and during the early stages of B cell activation. The final objective of these studies is to achieve some understanding of the apparently non-random association of antibody specificities with B lymphocyte subpopulations possessing unique triggering requirements during ontogeny. These studies will involve examination of the roles of sIgD and Lyb antigens in clonotype-restricted antibody responses which are confined to a mature subpopulation of B lymphocytes.